Not Applicable
1. Technical Field
The present invention relates to the field of airborne laser systems, and more specifically, to a laser system that detects obstacles and gathers air data information such as three-axis airspeed.
2. Background Art
It is known that a controlled moving mirror mounted one or more rotating axes can be used repetitively deflect a laser beam in such applications as laser printers, bar code scanners, and Light Detection and Ranging (LIDAR) systems.
Recent advancements in microeletromechanical systems (MEMS) have shown the potential for a scanner-on-a-chip. For example, a two-axis MEMS scanner device has been developed semiconductor manufacturing processes and a commercially successful Digital Micromirror Device (DMD) is available from Texas Instruments (TI).
This type of device, having a mirror no larger than 1 cm in diameter, enables the development of a compact light detecting and ranging (LIDAR) system. LIDAR system operation is known in the art, and a further description of such system operation is not included herein.
LIDAR systems can be broadly characterized as vehicle-mounted or fixed-mounted (i.e. stationary). A significant difference between the two relates to the environmental requirements. For example, airborne systems must operate through more severe temperature, vibration, and other extremes as compared to the more benign environment of stationary systems.
Unique to an airborne optical system is the choice of window materials that must withstand the abrasion effects of high-speed air streams.
One prior art LIDAR-based obstacle detection system goes by the tradename HELLAS, sold by Dornier GmbH of Friedrichshafen, Germany. According to its specification sheet, it uses a 1.54 micron laser, an InGaAs APD hybrid detector, a fiber-optic scanner for the horizontal direction, and an oscillating mirror in the vertical direction, with a scanning frequency of 2xcx9c4 Hz. It does not contain an air data mode.
There is a long-felt need for a compact, low-weight, low-cost LIDAR system that provides both obstacle detection and air speed determination. Such a dual mode system would provide a great benefit, especially for helicopter systems, where low air speed detection is difficult to measure.
The present invention comprises a unique scanning system for a dual-mode LIDAR system that detects obstacles in a first field of view (FOV) and also provides air data information from a second FOV. For example, the first FOV could be along the flight path, and the second FOV, in the case of a helicopter, can be up through the rotor blades, in an axis substantially orthogonal to the first FOV. This example will be used throughout the specification, although one skilled in the art can realize many other applications, some of which may benefit from the two field of views (FOV) not being orthogonal. Advantageously, my invention is suitable for installation on piloted-helicopters and/or uninhabited air vehicles (UAV) where weight, size and cost are critical system attributes.
The air data mode-of-operation is enabled by incorporating a fold mirror with a patterned aperture placed at the output of a laser scanner, in the general shape of a picture-frame, having a clear aperture through which the forward obstacle scanning laser beam passes, and a periphery with a reflective coating for intercepting a portion of the scan, deflecting those portions upward for air data collection.
It is anticipated that other patterns can be employed, for example, reflective portions placed only at the four corners of the fold mirror. In a further embodiment, an array of reflecting prisms, lens elements, or other optics is placed around the periphery of the fold mirror to direct the beam into a complex pattern of scan angles. Advantageously, this feature may be useful to detect air data in multiple regions above the aircraft. Alternatively, a region of the periphery of the fold mirror can direct scans towards the ground to determine height above terrain, and ground speed. This then would define scan patterns exiting the optical enclosure in three axes (upward, forward, and downward). Additionally, the central clear aperture can be patterned with reflective dots, lenslets, or prisms, to further add degrees of scanning freedom. In other embodiments, the clear aperture is made partially reflective to allow for simultaneous scans in multiple fields of view.
Note that the laser as referenced herein can use either visible light or non-visible light, the latter typically based on infrared (IR) lasers, with the preferred IR lasers operating in the eye-safe region.
The invention, then, relates to the use of a microscanner and a stationary fold mirror with a patterned aperture for scanning more than one axis. This allows the high-cost laser to provide several functions, thereby reducing overall cost as compared to an aircraft employing multiple separate laser systems.